To start, check out these handy templates which ROBLOX themselves has created. If you right click and save these images, you can use them as a guide on how the clothes should look once you upload ...

2 Users can make their own clothes with Roblox template Credit: AP How to make a Roblox shirt template Users can make their own clothes for their avatar and here we will show you the tricks needed.

Much like most things on Roblox, you will be making this yourself from scratch, so you'll need to design a t-shirt in an image and then upload it to Roblox.